I have never seen a wrong year on any cowl tag before. I have owned a November and January South Gate Jetfires, so before and after yours, but both had 63 on the tags. So it’s very strange.
One thing that would help, is to see the Protecto plate and book. It will have the selling date and engine numbers as well.
Then if we can see a photo of the engine numbers (right side head and block pad) to see if it all matches.
The car Would be great to restore.